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FOURTH QUARTER,2000
<br /> GROUNDWATER MONITORING REPORT
<br /> USA STATION#110-STOCKTON
<br /> JANUARY 2001
<br /> 3.1 Groundwater Flow and Gradient
<br /> Groundwater data collected at the site indicates that depths to groundwater ranged from 57.87- '
<br /> feet below ground surface (bgs) in MW-1 to 58.89-feet bgs in MW-6-P. The groundwater
<br /> elevations ranged from-32.26-feet below mean sea level(MSL) in MW-1 to -33.28-feet MSL in '
<br /> MW-6-P. Gauging data from MW-4P and MW-6P appear anomalous with over 8-feet of depth
<br /> difference from the previous quarter, the third quarter gauging data may be the anomalous
<br /> readings. These data were not used to calculate groundwater flow and gradient determination. ,
<br /> Repeatable results will determine the validity of this data.
<br /> Groundwater elevation measurements indicate groundwater was flowing to the southeast at a ,
<br /> hydraulic gradient of approximately 0.001-feet/feet at the time of this gauging event. A review of
<br /> the historical field data suggests that the groundwater generally flows to the east with minor
<br /> fluctuations to the north and south, as shown on Figure 5.
<br /> 3.2 Results of Laboratory Analyses
<br /> All petroleum compounds were non-detect (ND) at the method detection limit in MW-4-P. TPH
<br /> G was reported in monitoring wells MW-1, MW-2P, MW-3P, and MW-5P at 76 micrograms per
<br /> liter (,ug/1), 1,200,ug/l, 110 ,ug/l, and 120 4g/l, respectively. Benzene was detected in wells MW- '
<br /> 2P and MW-3P at 33 4g/1 and 0.72 ,ug/l. Toluene was reported in MW-1, MW-2, MW-1-P,
<br /> MW-2-P, MW-3-P, MW-5-P, and MW-6-P at 4.4 ,ug/l, 1.7 4g/l, 1.1 µg/l, 11 ug/l, 5.7µg11, 0.61
<br /> 4g/1, and 1.0 4g/I, respectively. Ethylbenzene was reported in MW-2P and MW-6-P at 2.0µg/1 '
<br /> and 1.3 Ag/l, respectively. Xylenes were also reported in MW-2-P and MW-6-P at 3.9 4g/l and
<br /> 0.61 4g/1, respectively. MTBE was ND by EPA Method 8020 in all wells sampled this quarter,
<br /> except MW-1, MW-2P and MW-3P where it was reported at 12 ,ug/l, 32 ,ug/l and 31 4g/l,
<br /> respectively.
<br /> 1,2-DCA was reported in five of the eight wells sampled(MW-1-P, MW-2-P, MW-3-P, MW-5-P,
<br /> and MW-6-P) at concentrations of 114g/1, 10 4g/l, 1.I 4g/l, 8.9 µg11, and 2.5 ,ug/l, respectively. '
<br /> Confirmation analyses for MTBE using EPA Method 8260B reported concentrations in MW-1,
<br /> MW-2-P and MW-3-P at 16 4g/l, 25 gg/l and 36 4g/I, respectively. DIPE was reported in
<br /> MW-3P, MW-5P and MW-6P at 5.7,ug/l, 110 4g/I and 11 ug/L respectively. TBA was reported '
<br /> in MW-2-P and MW-3P at 31 ,ug/l and 7.4,ug/l, respectively. All other samples were ND at the
<br /> method detection limit for this sampling event. The groundwater analytical results for this
<br /> sampling event are summarized in Tables 2 and 3, and shown on Figures 6 and 7. Copies of '
<br /> laboratory reports with COC documentation are included in Appendix B.
<br /> 3.3 Discussion ,
<br /> The appearance of DIPE in the perimeter wells (MW-3-P, MW-5-P and MW-6-P) and not in the
<br /> interior wells seem to support our earlier suspicions that the groundwater at this site is being ,
<br /> impacted by an off-site source, probably the former Chevron site across from Farmington Road.
<br /> 3 '
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